An inkjet printing system is characterized in that recording of images or characters is performed by attaching liquid microdroplets of an ink composition to a base material to be printed on (hereinafter, referred to as “printing base material”), and thereby no printing plate is used in the process of printing. As another printing system which does not use any printing plate, an electrophotographic system is also well known. However, from the viewpoints of equipment cost, running cost, printing speed and the like, the inkjet printing system is considered to be superior so that there has been an increase in the market demand for on-demand printing in recent years, and the demand is even further expanding.
As the ink compositions that are conventionally used in the inkjet printing system, a wide range of types such as a solvent type, an aqueous type, and an active energy ray-curable type are available. Among these, active energy ray-curable type ink compositions have an advantage that since the printed matter can be immediately cured and dried by irradiation of an active energy ray after printing, the printing speed is superior as compared with solvent type or aqueous type ink compositions, which essentially require an ink drying process. In addition to that, active energy ray-curable type ink compositions have many advantages, such as that there are available many base materials that can accommodate the use, such as non-absorptive base materials such as plastics and glass or paper base materials, and that the amounts of solvent volatilization are reduced, thus making the system environment-friendly. Therefore, it is the current situation that even in the inkjet printing system, the demand for the active energy ray-curable type ink compositions is ever increasing in recent years.
In order to cope with these demands, an active energy ray-curable ink composition is required to have drying properties and curability that are appropriate for realizing an excellent printing speed without depending on the print thickness of the printed matter or the coverage rate. Furthermore, an active energy ray-curable ink composition is required to have base material versatility for accommodating printing on various base materials. Also, an active energy ray-curable ink composition is required to have qualities such as non-yellowing properties for preventing any color change before and after curing. In order for an ink composition to be used for inkjet printing, it is an essential condition that the ink composition has a low viscosity.
However, while it is definitely natural to use a color ink composition in the inkjet printing system, there are occasions in which a colorless transparent clear ink composition is used in recent years. The purpose of using the clear ink composition may vary in a wide range, and the clear ink composition is used, for example, in the case of performing printing on top of a printed matter of color ink composition, and imparting a glossy feeling or a matte feeling to the surface of the printed matter to thereby enhance the design of the printed matter; in the case of printing characters with a high print thickness on the base material, and thereby producing a three-dimensional printed matter such as Braille; or in the case of directly performing printing in advance using a clear ink composition on a penetrable base material such as a paper base material, and then performing printing on top of a printed matter using a color ink composition to thereby prevent penetration into the base material, bleeding or the like and to obtain high quality images irrespective of the base material. Japanese Patent Application Laid-Open (JP-A) No. 2000-141708 discloses an example of the inkjet printing system including a clear ink composition.
There have been hitherto reports on the method for inkjet printing including a clear ink composition using various systems of inkjet ink compositions such as described above. For example, JP-A Nos. 2004-263049 and 2007-291399 discuss examples of inkjet printing methods of imparting glossiness to a printed matter by performing printing with an aqueous type clear ink composition on top of an aqueous type color ink printed matter. However, in the case of an aqueous type ink composition, since an ink drying step after printing is necessary as described above, there is a problem that the printing speed deteriorates.
Furthermore, JP-A No. 2007-031667 discloses an example of an active energy ray-curable type clear ink composition, by which it is intended to enhance the print quality by controlling the viscosity and surface tension of the ink composition and thereby preventing cissing on the color ink printed matter. However, the Examples describe only the case of performing printing on a color ink printed matter, and the case of directly printing on a base material as described above is not taken into consideration. Furthermore, the inventors of the present invention attempted a simulation experiment, which resulted in that the printed matter turned yellowish, and printed matters having poor non-yellowing properties were obtained.
As discussed above, it is the current situation that an active energy ray-curable inkjet ink composition, particularly a clear ink composition, which satisfies all of curability, base material versatility and non-yellowing properties, has not yet been obtained.